FIG. 7 is a perspective view illustrating a prior art semiconductor optical modulator. In FIG. 7, reference numeral 105 designates an InP substrate. A light absorbing layer 104 having barrier layers and two quantum wells between respective barrier layers is located in the InP substrate 105. An insulating film 103 of silicon dioxide is disposed on the InP substrate 105. A Cr/Au electrode 102 is in contact with the light absorbing layer 104. An Au layer 101 is disposed on the Cr/Au electrode 102.
The energy band diagram of the light absorbing layer 104 is described in, for example, Physical Review Letters, Vol. 60, pp. 2426 (1988), and will be described below.
FIG. 4 shows an energy band diagram of the light absorbing layer 4 when no electric field is applied to the light absorbing layer 4, and FIG. 5 shows an energy band diagram when an electric field is applied to the light absorbing layer 4.
In FIG. 4, reference numeral 10a designates a quantum well layer comprising InAlGaAs having a width W of 7 nm and reference numeral 10b designates a quantum well comprising InAlGaAs having a width W of 7 nm. Reference numeral 2 designates a barrier layer comprising InP having a width L of 3 nm. The energy band gap of the quantum well 10a is Eg(A.fwdarw.A), and the energy band gap between the valence band of the quantum well 10a and the conduction band of the quantum well 10b is Eg(A.fwdarw.B). Further, in FIG. 5, the energy band gap of the quantum well 10a when an electric field is applied is Eg'(A.fwdarw.A) and the energy band gap between the valence band of the quantum well 10a and the conduction band of the quantum well 10b when an electric field is applied is Eg'(A.fwdarw.B).
FIG. 6 shows absorption spectra of the quantum well in a case where an electric field is applied to the light absorbing layer and in a case where no electric field is applied thereto.
A description is given of the operation of this semiconductor optical modulator.
In the energy band diagram in a case where no electric field is applied to the light absorbing layer shown in FIG. 4, the energy band gap Eg(A.fwdarw.A) from the bottom level 1ha (=1hb) of the valence band of the quantum well 10a to the bottom level 1ea (=1eb) of the conduction band of the quantum well 10a and the energy band gap Eg(A.fwdarw.B) from the bottom level 1ha of the valence band of the quantum well 10a to the bottom level 1eb of the conduction band of the quantum well 10b are equal to each other, and the values are both 0.826 eV. Accordingly, as shown by a solid line in FIG. 6 for the absorption spectrum of the light absorbing layer, the absorption peaks of Eg(A.fwdarw.A) and Eg(A.fwdarw.B) overlap with each other at wavelength .lambda..sub.Eg =1500 nm.
When an electric field is applied to the light absorbing layer, as shown in FIG. 5, the bottom level 1ha of the valence band of the quantum well 10a increases its energy level with an increase in the electric field applied, while the bottom level of the conduction band thereof decreases with the increase in the electric field applied. As a result, the energy band gap Eg'(A.fwdarw.A) when an electric field is applied becomes 0.824 eV, which is lower than the energy band gap Eg(A.fwdarw.A)=0.826 eV that is obtained when no electric field is applied. Similarly as above, the bottom level 1eb of the conduction band of the quantum well 10b also decreases with an increase in the electric field applied, and the energy band gap Eg'(A.fwdarw.B) from the bottom level 1ha of the valence band of the quantum well 10a to the bottom level 1eb of the conduction band of the quantum well 10b becomes 0.811 eV, a smaller value than Eg'(A.fwdarw.A)=0.824 eV. As a result, the following relation stands: EQU Eg(A.fwdarw.A)=Eg(A.fwdarw.B)&gt;Eg'(A.fwdarw.A)&gt;Eg'(A.fwdarw.B).
Therefore, the absorption peak of Eg(A.fwdarw.A) and the absorption peak of Eg(A.fwdarw.B) respectively shift to the longer wavelength side when an electric field is applied thereto, the absorption spectrum shifts to the broken line in FIG. 6, and the wavelength .lambda.i of the absorption peak of the energy gap Eg'(A.fwdarw.B) when an electric field is applied thereto, becomes 1529 nm.
When light of wavelength coincident with the absorption peak of Eg'(A.fwdarw.B), that is, a light of wavelength 1529 nm is incident on the light absorbing layer having such an absorption spectrum, the light is absorbed more, by an amount .DELTA.a, when an electric field is applied than when no electric field is applied. Utilizing this effect, the amount of light transmitted through the light absorbing layer can be adjusted by turning on and off the electric field applied to the light adsorption layer, whereby the incident light is modulated and emitted as a digital signal.
In the prior art semiconductor optical modulator constituted as described above, although ideally no absorption occurs when no electric field is applied and a large absorption occurs when an electric field is applied, actually there is absorption when no electric field is applied as shown in FIG. 6, which in turn results in loss.